Sex difference in disease burden of inflammatory arthritis patients treated with tumor necrosis factor inhibitors as part of standard care

Objective Knowledge is needed on the total disease burden across the sexes in inflammatory arthritis (IA). We aimed to compare disease burden, including a broad range of health aspects, across men and women with IA treated with tumor necrosis factor inhibitors (TNFi). Methods Adult outpatients with IA (rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis) were included as part of standard care. Patient-reported outcomes, disease activity, TNFi trough levels, calprotectin, Work Productivity and Activity Impairment, comorbidities and cardiovascular risk profile were assessed. Unadjusted comparisons across sexes were done with independent t-test, Mann-Whitney U-test and X2-test and adjusted analyses with General Linear Models and logistic/ordinal logistic regression. Results A total of 305 IA patients were included (167 men, 138 women). A significantly lower proportion of women (45%) than men (59%) were in remission according to disease-specific composite scores (p = 0.02). Women had significantly worse scores on pain, joint pain, fatigue, enthesitis, Health Assessment Questionnaire and Short Form (SF)-36 vitality and social functioning (all p≤0.04). Both sexes had worse SF-36 scale scores than the general population. Women reported more absenteeism (work time missed) and activity impairment. TNFi trough levels, neutralizing antibodies and calprotectin were similar across sexes. A similar total number of comorbidities was seen. Self-reported hypothyroidism was more frequent in women. Men had higher 10-year calculated risk of fatal cardiovascular events. Conclusion Important differences in disease burden between men and women were seen. More attention to sex differences in the follow-up of IA patients is warranted.


Patients
In this cross-sectional, observational study a convenience sample of patients >18 years with IA (including RA, AS and PsA) currently under treatment with TNFi at the Department of Rheumatology, Sørlandet Hospital Kristiansand, were included from January 2016 to December 2017. The patients underwent assessment by a physician and nurse as part of standard care, and self-reported demographics and PROs in the computer system used for standard followup (GoTreatIT Rheuma1) [15]. Data on current and previous treatment were extracted from the GoTreatIt Rheuma database. The study was approved by the Norwegian Regional Committees for Medical and Health Research Ethics (2015/1196/REK midt). Written informed consent was obtained from each patient.

Comorbidities
Patients reported to have, or to have a history of, 20 different comorbidities, including high blood pressure, angina pectoris, heart attack, heart failure, bypass operation, intermittent claudication, stroke/brain hemorrhage/TIA, asthma/bronchitis/other pulmonary disease, allergy/hay fever/eczema, chronic back pain, cancer, neurological disease, diabetes, hypothyroidism, mental illness, problems with alcohol or drugs, kidney disease, liver disease, stomach ulcer and anemia (yes/no). Presence of parents or brothers or sisters with cardiovascular disease before 65 years of age for women and 55 years for men, was also reported (yes/no).

Cardiovascular risk
The Systematic Coronary Risk Evaluation project (SCORE) risk prediction algorithm was used to estimate the 10-year risk of a fatal cardiovascular event [31]. As recommended for Norway, we used the low risk version of SCORE [31]. The SCORE algorithm includes age, sex, smoking status, systolic blood pressure and total cholesterol/HDL ratio. Categorization into four SCORE groups were performed: Low risk (<1%), moderate risk (�1% and <5%), high-risk (�5% and <10%) and very high risk (�10%).

Statistics
Medians (25 th -75 th percentiles) were calculated for non-normally and means (SD) for normally distributed data. Proportions were presented as n (%). Unadjusted comparisons across sex were performed with Mann-Whitney U-test, independent t-test, Fisher's exact test or X 2 test, as appropriate.
Age-and diagnosis-adjusted comparisons across sex were performed with General Linear Models (continuous outcomes), logistic regression (binary outcomes) and ordinal logistic regression (ordinal outcomes). Comparison of remission, TNFi trough levels and neutralizing antibodies across sex were additionally adjusted for current use of csDMARDs and BMI. Radar diagrams were made for visualization of SF-36 scale scores. All analyses were performed as completer analyses. No correction for multiple comparisons were done. A p-value <0.05 was considered significant. Sensitivity analyses with additional adjustment for disease duration and BMI, in addition to age and diagnosis, were also performed. Statistical tests were done using SPSS for Windows V.26.0.0.1.

Results
We included a total of 305 patients with IA (120 RA, 108 AS and 77 PsA patients). One hundred and thirty-eight patients were women and 167 men ( Table 1).
The women were younger, had longer disease duration and lower BMI than the men. Similar proportions of men and women were smokers, whereas a higher proportion of men used snuff. The most common civil status was married, followed by cohabiter and single for men and cohabiter and divorced for women. Both men and women had a mean of 13 years of education. About three quarters of the patients were previously bionaïve with a higher proportion of men than women. Etanercept was the most commonly used TNFi, followed by infliximab, certolizumab, adalimumab and golimumab.

Patient-reported outcome measures of disease activity
Women reported significantly more pain and joint pain than the men, but similar levels of back pain (adjusted analyses) and morning stiffness ( Table 2).
Fatigue was also significantly worse in women than men. BASDAI and BASFI were similar across the sexes. As to patients' satisfaction with their IA disease activity, 72% of the patients    reported their condition to be much better or better, 12% unchanged and 16% worse or much worse. (Table 2). Patient satisfaction was similar across the sexes.

Other measures of disease activity
Composite scores of disease activity including DAS28CRP, DAS28ESR, CDAI, SDAI, DAPSA28 and ASDAS were all numerically higher in women than men, but only DAS28ESR was significantly worse in women. A significantly higher proportion of men than women were in remission in unadjusted (p = 0.02) as well as age-and diagnosis-adjusted analyses (p = 0.009, Table 2). Also with additional adjustment for current use of csDMARDs and BMI these differences were significant (p = 0.008). Investigator's global assessment of disease activity was similar across the sexes, whereas the enthesitis measure MASES was significantly higher in women. A similar proportion of men and women had erosions in hands and feet. BASMI was similar across the sexes.

Health-related quality of life
In adjusted analyses, women had significantly worse scores on the SF-36 vitality and social functioning scales than men, but similar SF-36 mental health, bodily pain, general health, physical functioning, role physical and role emotional scale scores, although several of these measures showed a trend versus worse values in women (Table 2). SF-36 PCS was significantly worse in women in unadjusted (p = 0.03) but not adjusted analyses (p = 0.06), whereas SF-36 MCS was similar across the sexes. In Fig 1 sex-specific SF-36 scale scores adjusted for age and diagnosis are visualized in a radar diagram, together with age-adjusted sex-specific scale scores from the Norwegian general population.

Physical functioning
Physical functioning measured by HAQ, was significantly better in men than in women. Both sexes reported similar levels of physical exercise. A majority exercised 1-2 times a week or 3 or more times a week. About a third did not exercise regularly and about 4% did not exercise due to disability or handicap.

Work status
Work status differed significantly across the sexes (Table 3).
Significantly higher proportions of men were currently in full-time work and women in part-time work. There were no clear differences in presenteeism and work productivity loss across the sexes, whereas absenteeism and activity impairment were worse in women (Table 3).

Laboratory markers, including TNFi trough levels, neutralizing antibodies and calprotectin
Women had significantly higher ESR than the men, but similar CRP, calprotectin and leukocytes in unadjusted, as well as age and diagnosis adjusted analyses (Table 2). Women and men had similar serum trough levels of infliximab, adalimumab, certolizumab and etanercept ( Table 2). Similar proportions of men and women had TNFi trough levels above the lower reference value. Eleven (4%) of the patients had neutralizing antibodies to TNFi, thereof 9 to infliximab, 1 to certolizumab and 1 to etanercept. Similar proportions of men and women had neutralizing antibodies.

Comorbidities
Information on self-reported comorbidities was available in 263 (144 men and 119 women) of the 305 patients. About a fourth of the patients reported no comorbidities. Further, about a third of the patients had 1 comorbidity, about a fifth 2 comorbidities and also about a fifth 3-4 comorbidities. The most common comorbidities were chronic back pain, high blood pressure and allergy/hay fever/eczema, followed by asthma/bronchitis/other pulmonary disease, stomach ulcer, mental illness, diabetes and hypothyroidism. A similar total number of comorbidities were seen across the sexes. Consistently in unadjusted and adjusted analyses, mental illness was more frequently reported by men and hypothyroidism by women.

Cardiovascular risk
Women had more frequently parents or brothers and sisters with coronary heart disease in young age (Table 4). Men had significantly higher systolic and diastolic blood pressure.
LDL was similar across the sexes, HDL and total cholesterol higher in women and triglycerides higher in men. HbA1c was similar across the sexes. The 10-year risk of a fatal cardiovascular event according to SCORE was significantly higher in men. A higher proportion of men than women had high/very high as well as moderate risk of a 10-year fatal cardiovascular event.

Sensitivity analyses
Sensitivity analyses with adjustment not only for age and diagnosis, but also for disease duration and BMI, did not change the significance of the findings, except for change from similar  For comorbidities, when also adjusting for BMI and disease duration in addition to age and diagnosis, selfreported mental illness changed from being significantly more frequently reported by men to being similar across the sexes (p = 0.05).

Discussion
In this observational, cross-sectional study of 305 patients with inflammatory arthritis treated with TNFi, we found important differences in disease burden between men and women. A significantly lower proportion of women than men were in remission according to disease-specific composite scores. Women reported significantly more pain, joint pain and fatigue and had higher MASES. Furthermore, women had significantly worse SF-36 vitality and social functioning scale scores than men. Both sexes had worse SF-36 scale scores, PCS and MCS compared with the Norwegian general population [25]. Physical functioning (HAQ) was better in men. Still, both sexes reported similar levels of physical exercise. Women were more often in part-time work and reported more absenteeism (work time missed) and activity impairment than the men. Proportions of patients with erosions in hands or feet, TNFi trough levels, neutralizing antibodies and calprotectin were similar across the sexes. A similar total number of comorbidities was seen for men and women. Hypothyroidism was more frequent in women.
Men had a less favorable cardiovascular risk profile than women and a higher 10-year risk of a fatal cardiovascular event.
In line with previous studies, is the finding of less women than men being in remission [7,11,12]. This finding was statistically significant in analyses adjusted for age and sex, in analyses additionally adjusted for csDMARD use and BMI as well as in sensitivity analyses, and in spite of all patients being currently under treatment with TNFi. All composite scores of disease  activity were numerically higher in women, although only DAS28-ESR reached statistical significance. ESR was higher in women (which has also been shown in the general population) and may explain the differences found between DAS28ESR and DAS28-CRP levels. Of note, in early phase clinical trials there is a persistent underrepresentation of women, despite accumulating evidence in support of sex-based differences in immune responses and prevalence of autoimmune diseases [32,33]. Proportions of patients with TNFi trough levels above the lower reference values and proportions of patients with neutralizing antibodies to TNFi, were similar for men and women in unadjusted analyses, analyses adjusted for age, sex, current csDMARD comedication and BMI as well as in sensitivity analyses. In accordance with other studies, most neutralizing antibodies were found against infliximab [34]. We had the opportunity to analyze calprotectin in serum and found similar levels of calprotectin across the sexes.
Although not routinely measured in most rheumatology clinics, calprotectin is found to be a promising marker of inflammation in patients with IA [35]. We found worse pain, joint pain, fatigue and MASES in women vs. men, which is in line with other studies [7,9,[11][12][13][14]. Interestingly, animal studies have shown that chronic pain in female mice, unlike in male mice, may additionally be mediated by adaptive immune cells, indicating that male mice cannot be used as proxies for females in pain research [36]. Psychological and social factors may also contribute to sex differences in pain [37]. Further, a lower proportion of women than men being in remission may also be of impact.
All SF-36 scale scores in our population of patients with IA were worse than in the Norwegian general population, which is in accordance with previous findings [38]. As to sex differences, all SF-36 scale scores were numerically worse in women, like also seen in the general population [25]. Women had significantly worse vitality and social functioning scale scores than men. Similar findings of worse vitality in women have previously been reported in a study of 271 Norwegian PsA patients [39]. Unlike our study, only adults aged 18-45 years were included in that study and no difference in social functioning across the sexes was found [39]. Our findings were confirmed in sensitivity analyses, in which also the SF-36 bodily pain and physical functioning scale scores were significantly worse in women.
Compared with the general population, we found SF-36 PCS to be more impaired than SF-36 MCS, which is in line with findings from the observational NOR-DMARD study on RA and PsA patients [38]. SF-36 PCS was somewhat better in our study than in the NOR-DMARD study, which included patients between 2000 and 2012, possibly reflecting the improved treatment options in 2016-2017 when our study was conducted [38]. Women had worse SF-36 PCS than men in unadjusted analyses as well as analyses adjusted for age, diagnosis, BMI and disease duration, but not when adjusting only for age and diagnosis. Women with IA have also in previous studies been found to have worse SF-36 PCS than men [9].
HAQ was worse in women, which is in line with previous reports [8,39,40]. Still, this did not seem to impact upon levels of physical exercise, which was similarly reported by the sexes. Overall, 61% of persons aged 66 and below were working. The common retirement age in Norway is 67 years, but some employees have a collective agreement of early retirement between the age of 62 and 67. In the general Norwegian population about 68% of men and 67% of women aged 18-74 years are currently working [41]. Significantly higher proportions of men were currently in full-time work and women in part-time work, similar to what is seen in the Norwegian general population [41]. Women experienced more absenteeism and activity impairment than men. A contributing factor to this might be the higher level of fatigue in the women, as fatigue has been found to contribute to work productivity impairment in patients with IA [42].
Comorbidities may impact upon treatment, as patients with specific comorbidities or multimorbidity may be more difficult to treat and less often achieve remission/the agreed treatment target [4,43]. No differences in number of comorbidities were seen across the sexes, neither in unadjusted analyses, nor in age-adjusted analyses or sensitivity analyses. Consistently across unadjusted and adjusted analyses, men more frequently self-reported mental illness and women more often hypothyroidism. However, sensitivity analyses did not confirm the finding of more frequent mental illness in men. A recent study has raised focus on the association between arthritis and autoimmune thyroid disorders and suggested regularly thyroid function follow-up particularly in women at high risk [44]. Studies have also raised focus on mental disorders like depression and anxiety, which may occur more often in patients with IA, with negative impact upon treatment outcomes [45].
Patients with IA have an increased risk of cardiovascular disease compared with the general population [46,47]. However, studies have shown that targeting inflammation with TNFi may decrease this risk [48]. In our study where all patients were under treatment with TNFi, we found a higher estimated 10-year risk of a fatal cardiovascular event in men than in women. Men had higher systolic and diastolic blood pressure, lower HDL and higher triglycerides, whereas level of LDL was similar across the sexes. Of note, at any given age, estimated 10-year risk of fatal cardiovascular disease according to SCORE is lower for women than men [49].
Limitations of the study include missing data for some of the outcomes, like often may be the case in observational studies. However, the proportion of missing data were in general acceptable, with e.g. about 15% missing for the different SF-36 measures. Further, as this was an observational study, no conclusions on causality may be drawn. We used disease-specific composite scores for assessment of remission, that is, DAS28<2.6 in RA, MDA in PsA and ASDAS<1.3 in AS patients. Still, a limitation of the study may be that we included different types of IA patients. However, we found consistent results also when adjusting for type of IA, as well as in sensitivity analyses.
The main strength of the study is that, to our knowledge, this is the study on sex differences in IA assessing the most comprehensive panel of health aspects, including not only PROs like HRQoL and pain measures, but also disease activity, TNFi trough levels, neutralizing antibodies, calprotectin, work status, physical functioning, comorbidities and cardiovascular risk. We believe that our study may raise awareness to the sex gap in reporting of clinical data, where more focus on women is warranted, to ensure better tailoring of treatment and follow-up also for women.
In conclusion, in this observational study on patients with IA treated with TNFi, less women than men were in a state of remission. Further, women had worse pain, joint pain, fatigue and enthesitis scores as well as worse SF-36 vitality and social functioning scale scores. TNFi trough levels, neutralizing antibodies and calprotectin were similar across the sexes. More men were in full-time and more women in part-time work. Women experienced more absenteeism and activity impairment. A higher proportion of women had a history of hypothyroidism. Men had a less favorable cardiovascular risk profile and a higher 10-year risk of a fatal cardiovascular event. Our study highlights that clinicians should be aware of sex differences when interpreting clinical outcomes and treatment responses in IA patients, especially when it comes to differences in perception of symptoms, e.g. pain and fatigue.